Low-density interconnected metal foams and methods of manufacture

ABSTRACT

Ultralow density pure metal foams, with density approaching 0.1% of the bulk density, and synthesis methods using interconnected metallic nanowires are provided. Nanowires of various sizes and metals are synthesized by electrodeposition into nanoporous templates such as anodized aluminum oxide or polycarbonate. The templates are etched away and the nanowires are dispersed into water through a suitable fluid exchange. Surface treatments ensure that nanowires remain sufficiently metallic and physically separated. Wire-water solutions can be dropped directly into liquid nitrogen in the form of droplets or placed into molds of various shapes. A freeze drying technique is employed to turn the resulting ice-wire mixture into a freestanding, low-density foam composed of interlocked nanowires. Finally, sintering or oxidation and reduction treatment of the foam material at elevated temperatures is used to connect the nanowires into an interconnected metallic foam, greatly improving the strength of the structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 111(a) continuation of PCTinternational application number PCT/US2016/064218 filed on Nov. 30,2016, incorporated herein by reference in its entirety, which claimspriority to, and the benefit of, U.S. provisional patent applicationSer. No. 62/261,211 filed on Nov. 30, 2015, incorporated herein byreference in its entirety. Priority is claimed to each of the foregoingapplications.

The above-referenced PCT international application was published as PCTInternational Publication No. WO 2017/095925 on Jun. 8, 2017, whichpublication is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support underBRCALL08-Per3-C-2-0006, awarded by the Defense Threat Reduction Agency;under DMR-1008791, awarded by the National Science Foundation; and underContract DE-AC04-94AL85000 between Sandia Corporation and the U.S.Department of Energy. The Government has certain rights in theinvention.

INCORPORATION-BY-REFERENCE OF COMPUTER PROGRAM APPENDIX

Not Applicable

BACKGROUND 1. Technical Field

The technology of this disclosure pertains generally to nanoscalestructure fabrication methods, and more particularly to anultralow-density, nanostructured, monolithic pure metal foams andfabrication methods.

2. Background Discussion

Nanoporous metal foams have a host of fascinating electrical, magnetic,mechanical, optical, thermal and chemical properties due to theirextremely high surface areas, nanoscale constricted geometries, and highporosity. They have critical potential applications in such fields ashigh energy density laser targets, lightweight materials, coatings,photovoltaics, heat exchangers and regenerators, trapping and targeteddrug delivery, etc.

Although a number of methods for preparing pure metal nanoparticlesexist, there are relatively few synthetic processes for producing bulk,monolithic forms of nanostructured metals that have been developed.Conventional methodologies for fabricating monolithic metallicnanoporous materials or foams have limited practical applications andare difficult to adapt to large scale production because of theircomplicated procedures and use of expensive materials.

One approach to fabricating nanoporous metal foams is by the selectivedealloying of a binary alloy which involves selectively etching aless-noble metal from a bimetallic alloy. The metallic alloy startingmaterial contains two or more elements, one of which is readilysusceptible to selective chemical or electrochemical etching thatpreferentially removes the element. The dealloying process involves theremoval of a substantial portion of the starting material to createporosity. For example, aluminum based alloys are often used withconventional chemical dealloying approaches because aluminum can beremoved from the alloy with either a strong acid or a strong base.

Although selective dealloying approaches have been effective forfabricating some nanoporous metallic structures, thediffusion-controlled processes of electrochemical or acid etching limitthe practical dimensions of the structure that can be formed.

In addition, the dealloying process also requires a starting alloy witha percolating network of pores or the formation of a network of pores tocomplete the etching process throughout the structure. Immersion timesin the acid or base are typically 2 to 5 days and times increase withthe size of the starting structure.

In some settings, full removal of the etched metal is necessary for thefunctionality of the final monolith structure. For example, residues ofthe selected etched metal of the starting alloy can greatly affect thecatalytic properties of the nanoporous foam. This further limits thesize and morphology of the starting structure and final metal foam.

Additionally, many metals are readily oxidized in air at roomtemperature resulting in the formation of oxides. Oxides may form on thesurfaces of the metal structure that may interfere with thefunctionality of the final foam.

Another approach to the production of nanoporous metal monolithsinvolves certain forms of combustion synthesis such as the thermaldecomposition of transition-metal complexes containing high nitrogenenergetic ligands. For example, nanostructured metal monolithic foamscan be formed with a self-propagating combustion synthesis processutilizing metal complexes of the energetic high-nitrogen ligand,bistetrazole amine (BTA) that are ignited in inert environments.Generally, the BTA metal complexes are prepared by the reaction ofmonohydrated bi(tetrazolato)amine or ammonium bi(tetrazolo)amine and aselected metal salt. The product is collected and dried as a finepowder.

This thermal decomposition process results in the formation of a bulkmaterial with nanoscale features in a matter of seconds. However, themicrostructural features of the resulting metal foams can varysignificantly depending on the composition and processing conditions.

Other approaches have used sacrificial templates of carbon or organicaerogels such as polysaccharide templates. However, these approaches areusually better suited for use with metal oxides.

Therefore, there is a need for fabrication methods for producingmechanically stable, ultralow-density, nanostructured, monolithic, metalfoams, that are facile, inexpensive, environmentally benign, andamenable to scale-up processing. The present technology satisfies theseneeds and is generally an improvement in the art.

BRIEF SUMMARY

The present technology provides methods for fabricating low cost,ultralow density pure metal foams, with tunable densities between 50%and 0.05% by volume of the bulk density, using interconnected metallicnanowires. The highly porous foam structures have scalable andmacroscopic overall sizes, in the range of several millimeters or more.Such materials will provide an unprecedented platform for exploration oflightweight materials, coatings, photovoltaics, thermoelectrics, heatexchangers, hydrogen storage and catalysts, and could havetransformative impacts in advanced materials and energy research.

The fabrication methods begin with the selection of the type of metalnanowires that are preferably made from pure metals such as Al, Ti, V,Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La,Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of thesemetals. Combinations of compatible metal nanowires can also be formed insome embodiments. The term nanowire is used in a general sense for ananoscale element and is intended to include a variety of structuressuch as nanotubes, nanorods, nanowires and nanoribbons etc., that areeither solid or hollow.

In one embodiment, the nanowires are formed with a template andelectrodeposition. The characteristics of the nanowires are determinedby the configuration of the template. The electrodeposition processallows the synthesis of a wide variety of metallic nanowires as thebuilding blocks for the foams. This is easily scalable for massproduction and is cost-effective, which are highly advantageous featuresparticularly compared to current lithography approaches in the art.

The selected metal nanowires are dispersed in a fluid that is suitablefor lyophilization such as deionized water. The concentration or densityof the nanowires within the freezable fluid can be controlled to tunethe density of the final foam. The dispersion may be prepared with thedensity of nanoscale metal wires tuned to a given application over acontinuous range from 0.05% to 50% by volume.

The outer surfaces of some metal nanowires may oxidize or have otherwisereacted with the environment during formation or in storage. Suchcontamination can be removed before the foam formation. For example,nanowires may be placed into a solution of an acid such an L-ascorbicacid to treat the wire surfaces and remove oxides and other contaminantsbefore being placed in the lyophilization fluid.

The tuned dispersion of treated nanowires is then used to fill shapedmolds for freezing. The molds can be essentially any shape and size. Inanother embodiment, the dispersion is dropped or injected directly intoa cryogenic liquid such as liquid nitrogen to form spheres, cylinders,discs, cubes, rectangular prisms, sheets, or other forms.

The frozen molded forms are then placed into a vacuum chamber forlyophilization. The frozen liquid sublimates under controlled conditionsleaving a structure of interlocked nanowires.

This structure of loosely interlocked nanowires is processed further tobond the points of contact between the nanowires to form the final foamwithout significant increase in density. The technology utilizes asintering step, the oxidation and reduction or sintering of thenanowires to transform the foam from an interlocked structure (where thenanowires are touching) into an interconnected structure (where thenanowires are further bonded). This greatly improves the strength of thematerial, allowing the formation of low and ultralow density metal foamsat 0.05% of the bulk density (e.g., 5 mg/cm³ Copper foams), which areuseful for a far wider range of applications than previously possible.

According to one aspect of the technology, interconnected pure metallicfoams with a wide range of tunable densities and characteristics isprovided. Aspects of the final metal foam can be tuned by the selectionof the length and the cross-sectional dimensions of the nanowireelements as well as the selection of the concentration or density of thenanowires in the nanowire dispersion.

Another aspect of the technology is to provide nanowire surfacetreatments using L-ascorbic acid to help ensure that the nanowiresremain sufficiently metallic, which facilitates the subsequent bondingprocesses during sintering.

An aspect of the technology is to provide a method for the creation ofthree dimensional, low and ultralow density foam structures throughelectrodeposition and a freeze-drying process

A further aspect of the technology is the creation of interconnectedpure metallic foams that have low density (down to 0.05% of its bulkdensity) and are still mechanically stable.

Another aspect of the technology is to provide a pure metal foam that isparticularly suited for producing x-ray emissions from laserillumination. For example, metallic foam structures with densities fromabout 20 g/cm³ to about 1 mg/cm³ are particularly useful as targets forhigh energy density lasers to generate bright x-rays.

Further aspects of the technology described herein will be brought outin the following portions of the specification, wherein the detaileddescription is for the purpose of fully disclosing preferred embodimentsof the technology without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The technology described herein will be more fully understood byreference to the following drawings which are for illustrative purposesonly:

FIG. 1 is a schematic flow diagram of a method of fabricating lowdensity interconnected pure metal foams according to one embodiment ofthe technology.

FIG. 2 is a schematic flow diagram of an alternative embodiment of themethod of fabrication of pure metal foams of interconnected nanowiresaccording to the technology.

FIG. 3 is an indentation curve with loading and unloading portions for a9 mg/cm³ strengthened Cu cylindrical foam that was 2 mm in height anddiameter. By measuring the initial slope of the unloading curve themodulus of the material can be extracted.

FIG. 4 is a relative modulus vs. relative density plot for various lowdensity materials as compared to the foams of the present technology.

DETAILED DESCRIPTION

Referring more specifically to the drawings, for illustrative purposes,embodiments of systems and methods for producing low-densityinterconnected metal foams are generally shown. Several embodiments ofthe technology are described generally in FIG. 1 through FIG. 4 toillustrate the fabrication system and methods. It will be appreciatedthat the methods may vary as to the specific steps and sequence and thesystems and apparatus may vary as to structural details withoutdeparting from the basic concepts as disclosed herein. The method stepsare merely exemplary of the order that these steps may occur. The stepsmay occur in any order that is desired, such that it still performs thegoals of the claimed technology.

Turning now to FIG. 1, a flow diagram of one embodiment of a method 10for fabricating low density metal foams with controllablecharacteristics is shown. In the step at block 20, the fabricationprocess generally begins with the preparation of a dispersion of metalnanowires in a freezable liquid.

A variety of fluids can be used to prepare the dispersion at block 20 ofFIG. 1. The fluid that is used is preferably one that does not reactwith the type of metal forming the metal nanowires that have beenselected for the foam. The preferred fluid will also freeze andsubsequently sublimate under controlled conditions and otherwise besuitable for lyophilization processes. Preferred fluids also will exertlimited stresses on the dispersed wires upon freezing. Deionized wateris particularly preferred but other solvents or pure substances thathave a triple point may also be suitable freezable fluids for wiredispersions, for example, liquid CO₂, iodine, arsenic, naphthalene andammonium chloride.

The metal nanowires that are provided at block 20 are preferably madefrom pure metals or metal alloys. Suitable metals for metal nanowirefoams generally include Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr,Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, andalloys based on one or more of these metals.

As used herein, the term “nanowire” generally refers to nanoscalestructures that have at least one cross-sectional dimension that is lessthan 1 μm, (i.e. in the nanoscale range), and preferably about 500 nm orless, and more preferably, 200 nm or less. Such nanoscale structureswill typically have an aspect ratio of the length to the smallestcross-sectional dimension of greater than 1, preferably, greater than10, and in many cases, greater than 100. However, in some embodiments,the nanoscale structures are nanorods with length and diameterdimensions that produce aspect ratios of between 2 and 10. Accordingly,both the length and the cross-sectional dimensions of the nanowireelement are variable.

The cross-section of the nanowire element may also have any arbitraryshape, including circular, square, rectangular, tubular, or elliptical,and may have a regular or an irregular shape. The nanoscale wire elementmay be solid or hollow. The term nanowire is intended to include avariety of structure such as nanotubes, nanorods, nanowires andnanoribbons etc.

The dispersion that is prepared at block 20 also has a nanowire densitythat can be selected. Wire density refers to number of nanowires perunit volume (e.g. μm³). The wire density of the final foam isdetermined, in part, by the concentration of nanowires in the nanowiredispersion volume that is initially prepared.

The dispersion may be prepared with essentially any density of nanoscalewires. However, the preferred volume ratio of the fluid in thedispersion vs. the nanowire volume, for example, ranges from 1 to10,000.

Therefore, some important aspects of the final metal foam can be tunedby the selection of the length and the cross-sectional dimensions of thenanowire elements as well as the selection of the concentration ordensity of the nanowires in the dispersion that is provided at block 20of the method of FIG. 1.

The surfaces of the nanowires in the dispersion that is provided may betreated to ensure that the nanowires remain sufficiently metallic atblock 30. This process is used to reduce any oxides that may be presentin on the surfaces of the nanowires. Wires are then moved back intodeionized water through fluid exchanges immediately prior to freezing atblock 40 of FIG. 1.

For example, the nanowires may be placed into a solution of an acid suchan L-ascorbic acid to treat the wire surfaces at block 30. For coppernanowires, the wires may be moved into 100 g/L L-ascorbic acid, forexample, where they are soaked for 48 hours in order to reduce any oxidein the copper to ensure the wires are pure copper during the foamformation process and to strengthen the van der Waals attractionsbetween the wires during the foam formation step.

However, this L-ascorbic acid treatment may not be necessary for silver,palladium, gold or platinum wires since they do not readily oxidize.Instead, the nanowire surface treatment at block 30 may be the immersionof the nanowires into a volume of 0.25 g/10 mL polyvinyl-pyrrolidone(PVP) surfactant to separate the wires. The treated wires are then movedback to water through a fluid exchange process. Alternative chemicalsfor such surface treatment may include oxalic acid for oxide removal andsurfactants are Cetrimonium bromide (CTAB), Sodium dodecyl sulfate(SDS), and polyvinyl alcohol (PVA) for nanowire separation.

Lyophilization techniques may be employed at block 40 and block 50 toturn the resulting ice-wire structure into a freestanding, low-densityfoam composed of interlocked nanowires. In order to create a nanowirestructure from the resulting wire-water solution at block 30, the wiresmust undergo a freeze-drying process to avoid collapse from the surfacetension of the water. Once the wire-water solution has been created, thedensity of the final foam structure can be adjusted by changing thewater to wire ratio.

In one embodiment, the dispersion of treated nanowires and freezablefluid may be placed into various shaped molds and frozen at block 40 ofthe method of FIG. 1. To make foams of selected shapes, thin coppermolds (for high thermal conductivity) may be used to freeze nanowiresolutions into the shapes of the molds.

To create spherical ice-wire structures and foams, wire-water solutionscan also be dropped directly into liquid nitrogen or other cryogenicliquid in the form of droplets and then frozen as spheres at block 40.Pipettes of varying diameters can be used to adjust the diameters of thespherical ice-wire structures.

Lyophilization techniques that are employed at block 50 normally work byreducing the surrounding pressure of wire-liquid material frozen atblock 40, for example in a vacuum, to allow the frozen water in thematerial to sublimate (i.e. move directly from the solid phase to thegas phase).

Commercial lyophilization devices generally use a four-step process: 1)pretreatment; 2) freezing; 3) primary drying; and 4) secondary drying.In the freezing step, the material is cooled to a temperature that isbelow the lowest temperature where the solid phase and liquid phase cancoexist, called the triple point. This cooling ensures that the materialwill sublimate rather than melt during the drying steps.

In the primary drying phase, the pressure is lowered and the materialmay be heated to an optimal point where the frozen material sublimatesefficiently. Typically approximately 95% of the water sublimates away inthe primary drying phase.

During the secondary drying phase, the pressure may be lowered furtherand the material temperature may also be increased to facilitatedesorption of remaining water molecules from the material.

The freeze-drying steps of block 50 of FIG. 1 leave a free-standingstructure of interlocked nanowires. This interlocked structure isprocessed further at block 60 of FIG. 1. The process changes thestructure from a loosely interlocked lattice of wires into a singleinterconnected pure-metal foam without a substantial increase indensity. At block 60, the ice-wire structure preferably undergoesseveral oxidation-reduction cycles to physically coalesce the wirestogether at the intersections to form the final foam.

Referring now to FIG. 2, a flow diagram of an alternative embodiment ofa method 70 for fabricating low density metal foams with controllablecharacteristics is shown. The method 70 described in FIG. 2 begins withthe formation of a nanoporous membrane template at block 80. Thismembrane template has pores or channels with controllable dimensionsthat provide a template for the creation of nanowires. Nanowires can begrown by electrodeposition into nanoporous templates of various sizesand types. The preferred template materials include anodized aluminumoxide (AAO) membranes, polycarbonate membranes, porous mica membranes ornanochannel glass templates, to lengths of between approximately 1 μm to100 μm and diameters from approximately 10 nm to 1,000 nm.

At block 90, a working electrode is coated onto one side of the startingmembrane that was fabricated or provided at block 80. For example, ametal layer may be coated onto one side of the membrane by a variety ofprocedures such as by magnetron sputtering to cover the template poresand to be used as a working electrode during electrodeposition.

Nanowires are then formed by electrodeposition in the membrane at block100. One preferred electrodeposition process is carried out in athree-electrode cell with electrolytes tailored to the particular metalthat is to be deposited in the pores and channels of the template.

After electrodeposition at block 100, the working electrode on themembrane template deposited at block 90 is removed at block 110. Theworking electrode may be removed by conventional etching procedures atblock 110 that will dissolve the electrode.

The nanowires are then released from the membrane template bydisintegration of the template at block 120. The nanowires can beseparated from template material by both chemical and mechanical methodssuch as by etching and sonication.

At block 130 of FIG. 2, the released nanowires may be optionallyagitated to separate and randomize the nanowires. It was found thatduring this template removal process the wires tended to clump together(long axes parallel to one another) due to van der Waals attractionsbetween the wires. To disrupt this clumping, the containers could beshaken aggressively by hand or sonicated.

The surfaces of the nanowires are then treated at block 140 to eliminatethe presence of any oxides or other materials that may appear on thesurface of the nanowires during the fabrication steps. One surfacetreatment is with a mild acid such as ascorbic acid that will removeoxides formed on the surface. The nanowire surface treatments at block140 may also be in the form of exposure to a surfactant.

The treated nanowires are then washed and dispersed in a suitable fluidfor freeze drying (lyophilization) at block 150. Washing removes theunused surface treatments and other contaminants. The washing anddispersion step at block 150 can also include concentrating or dilutingthe nanowires to a desired density, which will influence thecharacteristics of the final foam that is produced.

At block 160 of the method of FIG. 2, the prepared dispersion of treatednanowires is then frozen. Optionally, the dispersion can be placed inshaped molds to produce foams with a selected shape. Spherical shapedfoams can also be produced by placing drops of nanowire-fluid mixtureinto a cryogenic liquid such as liquid nitrogen.

Lyophilization of the frozen wire-fluid mixture at block 160 willproduce an interlocked nanowire structure at block 170. ThisLyophilization procedure can also be performed in commercially availablefreeze-drying machines.

The interlocked wire structure that remains after the removal of thefrozen fluid at block 170 is processed further to form an interconnectedpure-metal foam at block 180 of the method of FIG. 2. The points ofcontact between nanowires of the interlocked nanowire structure arepreferably sintered by heating the interlocked nanowire structurewithout melting the wires to the point of liquefaction or by exposingthe structure to several oxidation-reduction cycles.

The technology described herein may be better understood with referenceto the accompanying examples, which are intended for purposes ofillustration only and should not be construed as in any sense limitingthe scope of the technology described herein as defined in the claimsappended hereto.

Example 1

In order to demonstrate the operational principles of the fabricationmethods, several metal foams were prepared using the fabrication methodshown schematically in FIG. 2. In this example, pure metal nanowirefoams of Cu, Pd, Co and Ag were fabricated with a wide range of tunabledensities.

Nanowires were grown by electrodeposition into nanoporous templates ofvarious sizes and types, including anodized aluminum oxide (AAO)membranes and polycarbonate membranes, to lengths of between 5 μm and 40μm and diameters from about 50 nm to 200 nm.

Magnetron sputtering was used to coat thick conductive layers (200 nm to500 nm) on the backsides of the membranes for use as working electrodes.For copper and cobalt nanowires, a 500 nm thick copper layer was coatedonto the backside of the membrane to cover the template pores and to beused as a working electrode during electrodeposition. For silver,palladium, gold and platinum nanowires, a 500 nm gold working electrodewas used instead.

These coated membranes were then placed into an electrodeposition cellwhere metals were deposited potentiostatically into the pores to grownanowires. The electrodeposition of wires was carried out in athree-electrode cell with suitable electrolytes. In all cases, aplatinum counter electrode was used along with an Ag⁺/AgCl referenceelectrode.

Potential was pulsed to help ensure uniform deposition (for example,from 0 mV to −200 mV at one second intervals for copper deposition). Thedeposition current was monitored and growth was halted when an increasein current corresponding to the onset of over-deposition was detected.Deposition potential was pulsed and electrolytes containing largeamounts of metal ions were used to ensure deposition uniformity. Forcopper nanowires, the deposition was carried out with an electrolytecomposed of 238 g/L copper sulfate and 21 g/L sulfuric acid. For silvernanowires the electrolyte was composed of 15.6 g/L silver sulfate and224 g/L potassium thiocyanate. For palladium nanowires the depositionelectrolyte was composed of 5 g/L PdCl₂ and 10 g/L HCl. Finally, forcobalt nanowires the electrolyte was composed of 50 g/L cobalt sulfateand 40 g/L boric acid.

Following deposition, the working electrodes of the templates wereremoved by floating the wire-filled AAO (working electrode side down) inan etchant solution. For a copper working electrode, 7.9M nitric acidwas used for 15 seconds. For a gold working electrode, a solutioncontaining 10 mL of water, 4 grams of potassium iodide and 2 grams ofiodine was used.

The wire-filled membrane was then thoroughly rinsed in deionized water.The template was then immersed in 6M NaOH and sonicated for 15 minutesto dissolve the AAO template and remove the nanowires. It was found thatduring this template removal process the wires tend to clump together(long axes parallel to one another) due to van der Waals attractionsbetween the nanowires. To prevent this clumping it was necessary toaggressively shake by hand the NaOH-nanowire solution periodically(e.g., at 5 minutes, 10 minutes and directly at the end of thesonication).

Following this process, multiple fluid exchanges were used to move thenanowires into deionized water. Average wire diameter of 150 nm and anaverage length was 20 μm were found using SEM.

For copper nanowires, the same exchange process was then used totransfer the wires into 100 g/L L-ascorbic acid where they were soakedfor 48 hours in order to reduce any oxide in the copper to ensure thewires are pure copper to strengthen van der Waals attractions betweenthe wires during the foam formation step. Wires are transferred backinto deionized water through fluid exchanges immediately prior to foamformation. The Co, Ag and Pd wires were instead immersed in 250 g/L PVPto separate the wires, which were then transferred back to water throughthe fluid exchange process.

In order to create a nanowire foam from the resulting wire-watersolution, the wires must undergo a freeze-drying process to avoidcollapse from the surface tension of the water. Once the wire-watersolution has been created, the density of the final foam can be adjustedby changing the wire-water ratio. Densities from 200 mg/cm³ to 8 mg/cm³were achieved using this process. To create spherical foams, droplets ofthis solution were placed directly into liquid nitrogen and frozen asspheres. Pipettes of varying diameters were used to adjust the diametersof foams from 2 mm to 5 mm. Ice-wire spheres were then moved into smallbaskets composed of a single loop of thin (127 μm diameter) wire tosupport the structure during freeze-drying. This wire was always made ofthe same material as the foam. To make foams of other shapes, thincopper molds (for high thermal conductivity) were used to freezenanowire solutions into the shapes of the rectangular and cylindricalmolds. Frozen foams were placed in rough vacuum (˜10 mTorr) in order tosublimate out the ice from the sample, leaving a free-standing nanowirestructures of interlocked nanowires.

An additional process was used to strengthen the metal nanowire foams.Spherical and cylindrical foams were transported into a tube furnace andunderwent oxidation-reduction cycles to physically coalesce wirestogether at intersections, changing the structure from a looselyinterlocked lattice of wires into a single interconnected pure-metalfoam without a significant increase in density. For copper nanowires,the foams were placed first in air at 300° C. for 20 minutes in order tooxidize them completely. Next they were reduced back to pure copper byputting them in a forming gas (5% hydrogen in nitrogen) for 20 minutesat 300° C. Oxygen kinetics and copper atom mobility during theseprocesses allow the foams to form into interconnected pure-metalstructures.

Example 2

Nanoindentation experiments were carried out on the foams produced withthe methods shown in Example 1 to quantify the enhanced strength of theinterconnected Cu nanowire foams. In order to perform nanoindentationmeasurements, cylindrical Cu foams with a 2 mm height and diameter werefabricated and strengthened using the oxidation/reduction process. Thefinal foams ranged in densities from 8 mg/cm³ to 70 mg/cm³. The indentertip was a 2 mm ruby sphere and measurements were conducted at roomtemperature and ambient laboratory humidity. The loading and unloadingrate was kept constant at 100 μm/m. A sample loading/unloading curve isshown in FIG. 3.

Loading/unloading curves were derived for a 9 mg/cm³ strengthened Cucylindrical foam with 2 mm in height and diameter. By measuring theinitial slope of the unloading curve it was possible to extract themodulus of the subject material. The elastic modulus of each sample wasextracted from the initial slope of the unloading curve using theOliver-Pharr multi-point unloading method. For a spherical indenter tipcontacting a flat surface the Elastic Modulus, E^(r) can be calculatedas:

$E^{r} = \frac{S\sqrt{\pi}}{2\sqrt{A( h_{contact} )}}$

where S is the unloading stiffness of the sample,

$\frac{dP}{dh},$

taken here as the slope of the first 15% of the unloading curve of eachsample; A(h_(contact)) is the area of contact between the indenter tipand the sample at the depth of penetration below the plane of contact:

A(h _(contact))=2πRh _(contact)−π(h _(contact))²

and h_(contact) was calculated as:

$h_{contact} = {h - {\frac{3}{4}\frac{P}{S}}}$

where P is the load on the sample and h is the depth of penetration.

FIG. 4 is a relative modulus vs. relative density plot for various lowdensity materials. The graph of FIG. 4 includes the results of the Cumeasurements as well as results from previous measurements of low andultralow density materials.

Plotted here are the relative density (defined as the final density ofthe material normalized by the bulk density of the material from whichthey are constructed) and the relative modulus (the elastic modulus ofthe sample normalized by the bulk modulus of its constituent material)of such samples.

The results shown as open circles in FIG. 4 represent interlocked copperwire foams from previous results in the art for comparison. At 9 mg/cm³,for example, the prior work shows a modulus of about 5 Pa. At the samedensity, the interconnected foams structure made from the same materialproduced by the current methods was found to have a modulus of 1200 Pa,an improvement in elastic modulus of three orders of magnitude at thesame density.

Also depicted in FIG. 4 are several other low and ultralow densitymaterials whose modulus has been measured through nanoindentation forcomparison. Using the methods of the present technology it was possibleto create a pure metallic foam whose relative modulus is comparable tostrong materials such as C and Si aerogels at the same relativedensities. Note that at similar relative densities Ni microlattices havea much greater relative modulus than the interconnected Cu foams, butrequire an elaborate nanolithography process. The nanowire foamsproduced by the methods described herein can be fabricated over largeareas and volumes much more easily and cost-effectively as the methodsrely only on electrodeposition and the samples are fabricated inside ofmacroscopic molds.

Example 3

One potential use of ultralow density metal foams is for use as highenergy density laser targets due to their ability to be heatedvolumetrically that allows targets composed of high-Z elements touniformly reach the extreme temperatures that are required for X-rayemission. Gas targets, oxides, metal-doped aerogels and metal-linedcavities have been employed previously for reaching the necessaryeffective density. However, the pure metal targets allow higher X-rayconversion efficiencies if the targets can be fabricated at densitieslow enough to allow volumetric heating while still maintainingmechanical stability.

To demonstrate the functionality of pure metal foam targets produced bythe methods, spherical Cu targets (2 mm and 4 mm diameters) andcylindrical Ag targets (4 mm diameter and length) were produced to beused as targets for testing at the OMEGA and the National IgnitionFacility (NIF) laser facilities. The enhanced mechanical strength wascritical to the use of such metal foams for use as x-ray emissionmaterials.

At the OMEGA facility each of 40 beams delivered 475 J of 351 nm laserlight to the spherical foam sample. Of these beams, 20 beams wereincident on each hemisphere of the spherical Cu targets and were focused600 μm inside the sample.

In the OMEGA demonstration, the volumetric heating of the Cu foamsallowed for substantially higher K-shell energy conversion efficiencythan the solid copper reference, reaching a maximum of 2.59% for the Cufoam with a 22 mg/cm³ density.

At the NIF facility all 192 beams were used, delivering a total of 500kJ of energy to the sample with 351 nm laser light, and 96 beams werefocused 550 μm inside each hemisphere. Spherical samples were mounted onthe end of small sticks with a support ring also made of AWG 36 copper(99.9%).

In the NIF demonstration, time-resolved X-ray emission data for energyintegrated in the Cu K-shell was obtained from three different foamdensity samples. The results obtained from Cu foams with densities of 10mg/cm³ to 18 mg/cm³ showed increasing conversion efficiency as thedensity was increased.

Accordingly, the pure Cu metal foams with density around 20 mg/cm³ andbelow have been successfully shot at OMEGA and NIF facilities, where thelaser energy has allowed the foams to be heated volumetrically and toproduce high X-ray conversion efficiencies. This opens up an unchartedterritory to explore high-Z plasma physics.

In summary, ultralow density Cu, Ag, Co and Pd pure metal foams havebeen fabricated using nanowire starting material (made byelectrodeposition or other methods) and a freeze-drying process. Thedensities of the foams can be tuned from as low as 8 mg/cm³, or 0.09% ofthe bulk density, to as high as 200 mg/cm³ and beyond.

Furthermore, it was shown that the Cu nanowire foams can be greatlystrengthened through an oxidation and reduction process by transforminga loosely interlocked structure into a truly interconnected nanowiremonolith. The elastic modulus of such foams was found to increase bythree orders of magnitude compared to that of an interlocked structure.Such mechanically stable pure metal foams open up a wide range ofapplications, including high energy density laser targets.

Embodiments of the present technology may be described herein withreference to flowchart illustrations of methods and systems according toembodiments of the technology, and/or procedures, algorithms, steps,operations, formulae, or other computational depictions, which may alsobe implemented as computer program products. In this regard, each blockor step of a flowchart, and combinations of blocks (and/or steps) in aflowchart, as well as any procedure, algorithm, step, operation,formula, or computational depiction can be implemented by various means,such as hardware, firmware, and/or software including one or morecomputer program instructions embodied in computer-readable programcode.

Accordingly, blocks of the flowcharts, and procedures, algorithms,steps, operations, formulae, or computational depictions describedherein support combinations of means for performing the specifiedfunction(s), combinations of steps for performing the specifiedfunction(s), and computer program instructions, such as embodied incomputer-readable program code logic means, for performing the specifiedfunction(s). It will also be understood that each block of the flowchartillustrations, as well as any procedures, algorithms, steps, operations,formulae, or computational depictions and combinations thereof describedherein, can be implemented by special purpose hardware-based computersystems which perform the specified function(s) or step(s), orcombinations of special purpose hardware and computer-readable programcode.

From the description herein, it will be appreciated that that thepresent disclosure encompasses multiple embodiments which include, butare not limited to, the following:

1. A method for fabricating low density and ultralow densitynanostructured metal foams, the method comprising: (a) forming adispersion of metal nanowires in a freezable fluid; (b) freezing theliquid dispersion to form an ice-nanowire structure; (c) sublimating thefrozen liquid to expose a free standing nanowire foam; and (d) bindingthe nanowire foam structure at points of contact to form aninterconnected metal foam monolith.

2. The method of any preceding embodiment, further comprising: treatingnanowire surfaces with an acid to remove oxide contaminants; anddispersing treated nanowires in a freezable liquid.

3. The method of any preceding embodiment, further comprising: treatingnanowire surfaces with a surfactant to separate nanowires from eachother; and dispersing treated nanowires in a freezable liquid.

4. The method of any preceding embodiment, wherein the metal nanowiresare formed from a metal selected from the group of metals consisting ofAl, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag,Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one ormore of these metals.

5. The method of any preceding embodiment, wherein the metal nanowireshave an aspect ratio of length vs. diameter within the range of 2 to1,000,000.

6. The method of any preceding embodiment, wherein the dispersioncontains wires diluted in volume by a factor of 2 to 10,000.

7. The method of any preceding embodiment, further comprising:depositing the liquid dispersion in one or more molds; and freezing theliquid dispersion in the molds.

8. The method of any preceding embodiment, wherein the binding of pointsof contact of the ice-nanowire structure is performed by sintering.

9. The method of any preceding embodiment, wherein the binding of pointsof contact of the ice-nanowire structure is performed by one or moreoxidation and reduction cycle(s) performed at elevated temperatures.

10. A method for fabricating low density and ultralow densitynanostructured metal foams, the method comprising: (a) preparing ananoporous membrane template; (b) applying an electrode to one side ofthe nanoporous membrane template: (c) forming nanowires within thenanoporous membrane template by electrodeposition; (d) releasing theformed nanowires from the membrane template; (e) forming a dispersion ofmetal nanowires in a freezable fluid; (f) freezing the liquiddispersion; (g) sublimating the frozen liquid to expose an interlockednanowire structure; and (h) binding the interlocked nanowire structureat points of contact between nanowires to form an interconnected metalfoam.

11. The method of any preceding embodiment, wherein the releasing offormed nanowires comprises: etching the membrane template to remove theelectrode; disintegrating the membrane template to release nanowires;dispersing the released nanowires in a freezable fluid; and agitatingreleased nanowires to separate and randomize nanowires in the freezablefluid.

12. The method of any preceding embodiment, wherein the nanoporousmembrane template comprises an anodized aluminum oxide (AAO) membrane, apolycarbonate membrane, a porous mica membrane or a nanochannel glassmembrane.

13. The method of any preceding embodiment, further comprising: treatingnanowire surfaces with an acid to remove oxide contaminants; anddispersing treated nanowires in a freezable liquid.

14. The method of any preceding embodiment, further comprising: treatingnanowire surfaces with a surfactant to separate nanowires from eachother; and dispersing treated nanowires in a freezable liquid.

15. The method of any preceding embodiment, further comprising:depositing the liquid dispersion in one or more molds; and freezing theliquid dispersion in the molds.

16. The method of any preceding embodiment, wherein the metal nanowiresare formed from a metal selected from the group of metals consisting ofAl, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag,Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, and alloys based on one ormore of these metals.

17. The method of any preceding embodiment, wherein the metal nanowireshave an aspect ratio within the range of 2 to 1,000,000.

18. The method of any preceding embodiment, wherein the dispersionwherein the dispersion contains wires diluted in volume by a factor of 2to 10,000.

19. The method of any preceding embodiment, wherein the binding ofpoints of contact of the ice-nanowire structure is performed bysintering.

20. The method of any preceding embodiment, wherein the binding ofpoints of contact of the ice-nanowire structure is performed by one ormore oxidation and reduction cycle(s) performed at elevatedtemperatures.

21. A metallic foam structure comprising; (a) an interconnected metallicnanowire network; (b) wherein the network structure has a density ofabout 0.1% of the bulk density.

22. The metallic foam structure of any preceding embodiment, wherein thenetwork structure has a density from about 20 g/cm³ to about 1 mg/cm³.

23. The method of any preceding embodiment, wherein the metal nanowirenetwork is formed from one or more metals selected from the group ofmetals consisting of Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Nb,Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy, Pt, Au, Pb, and Bi, andalloys based on one or more of these metals.

Although the description herein contains many details, these should notbe construed as limiting the scope of the disclosure but as merelyproviding illustrations of some of the presently preferred embodiments.Therefore, it will be appreciated that the scope of the disclosure fullyencompasses other embodiments which may become obvious to those skilledin the art.

In the claims, reference to an element in the singular is not intendedto mean “one and only one” unless explicitly so stated, but rather “oneor more.” All structural, chemical, and functional equivalents to theelements of the disclosed embodiments that are known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed as a “means plus function”element unless the element is expressly recited using the phrase “meansfor”. No claim element herein is to be construed as a “step plusfunction” element unless the element is expressly recited using thephrase “step for”.

What is claimed is:
 1. A method for fabricating low density and ultralowdensity nanostructured metal foams, the method comprising: (a) forming aliquid dispersion of metal nanowires in a freezable fluid; (b) freezingthe liquid dispersion to form an ice-nanowire structure; (c) sublimatingthe ice-nanowire structure to expose a free standing nanowire foamstructure; and (d) binding the nanowire foam structure at points ofcontact to form an interconnected metal foam monolith.
 2. The method ofclaim 1, further comprising: treating nanowire surfaces with an acid toremove oxide contaminants; and dispersing treated nanowires in afreezable liquid.
 3. The method of claim 1, further comprising: treatingnanowire surfaces with a surfactant to separate nanowires from eachother; and dispersing treated nanowires in a freezable liquid.
 4. Themethod of claim 1, wherein said metal nanowires are formed from a metalselected from the group of metals consisting of Al, Ti, V, Cr, Fe, Co,Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy,Pt, Au, Pb, and Bi, and alloys based on one or more of these metals. 5.The method of claim 1, wherein said metal nanowires have an aspect ratioof length vs. diameter within the range of 2 to 1,000,000.
 6. The methodof claim 1, wherein said dispersion contains wires diluted in volume bya factor of 2 to 10,000.
 7. The method of claim 1, further comprising:depositing the liquid dispersion in one or more molds; and freezing theliquid dispersion in the molds.
 8. The method of claim 1, wherein saidbinding of points of contact of the ice-nanowire structure is performedby sintering.
 9. The method of claim 1, wherein said binding of pointsof contact of the ice-nanowire structure is performed by one or moreoxidation and reduction cycle(s) performed at elevated temperatures. 10.A method for fabricating low density and ultralow density nanostructuredmetal foams, the method comprising: (a) preparing a nanoporous membranetemplate; (b) applying an electrode to one side of the nanoporousmembrane template: (c) forming nanowires within the nanoporous membranetemplate by electrodeposition; (d) releasing the formed nanowires fromthe membrane template; (e) forming a liquid dispersion of metalnanowires in a freezable fluid; (f) freezing the liquid dispersion toform an ice-nanowire structure; (g) sublimating the ice-nanowirestructure to expose an interlocked nanowire structure; and (h) bindingthe interlocked nanowire structure at points of contact betweennanowires to form an interconnected metal foam.
 11. The method of claim10, wherein said releasing of formed nanowires comprises: etching themembrane template to remove the electrode; disintegrating the membranetemplate to release nanowires; dispersing the released nanowires in afreezable fluid; and agitating released nanowires to separate andrandomize nanowires in the freezable fluid.
 12. The method of claim 10,wherein said nanoporous membrane template comprises an anodized aluminumoxide (AAO) membrane, a polycarbonate membrane, a porous mica membraneor a nanochannel glass membrane.
 13. The method of claim 10, furthercomprising: treating nanowire surfaces with an acid to remove oxidecontaminants; and dispersing treated nanowires in a freezable liquid.14. The method of claim 10, further comprising: treating nanowiresurfaces with a surfactant to separate nanowires from each other; anddispersing treated nanowires in a freezable liquid.
 15. The method ofclaim 10, further comprising: depositing the liquid dispersion in one ormore molds; and freezing the liquid dispersion in the molds.
 16. Themethod of claim 10, wherein said metal nanowires are formed from a metalselected from the group of metals consisting of Al, Ti, V, Cr, Fe, Co,Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm, Dy,Pt, Au, Pb, and Bi, and alloys based on one or more of these metals. 17.The method of claim 10, wherein said metal nanowires have an aspectratio within the range of 2 to 1,000,000.
 18. The method of claim 10,wherein said liquid dispersion contains wires diluted in volume by afactor of 2 to 10,000.
 19. The method of claim 10, wherein said bindingof points of contact of the interlocked nanowire structure is performedby sintering.
 20. The method of claim 10, wherein said binding of pointsof contact of the interlocked nanowire structure is performed by one ormore oxidation and reduction cycle(s) performed at elevatedtemperatures.
 21. A metallic foam structure having a bulk density andcomprising: (a) an interconnected metallic nanowire network structure;(b) wherein the network structure has a density of about 0.1% of thebulk density.
 22. The metallic foam structure of claim 21, wherein thenetwork structure has a density from about 20 g/cm³ to about 1 mg/cm³.23. The metallic foam structure of claim 21, wherein said metallicnanowire network structure is formed from one or more metals selectedfrom the group of metals consisting of consisting of Al, Ti, V, Cr, Fe,Co, Ni, Cu, Zn, Ga, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Sn, Sb, La, Nd, Sm,Dy, Pt, Au, Pb, and Bi, and alloys based on one or more of these metals.